1. Field of the Present Invention
The present invention relates in general to solid state electronic amplifiers, and more particularly to low cost audio power amplifiers with ultra low distortion and high power efficiency performance for audio systems. The present invention further relates to a method of using two kinds of new local Negative Feedback (NFB) networks to replace the traditional Miller compensation capacitor used in various amplifier circuits, including most commonly used IC (integrated circuit) operational-amplifiers, for improving the performance of amplifiers over the extended frequency band which users have the most interest in and put growing emphases on for their applications.
2. Description of Related Arts
An electronic amplifier is employed to make signals stronger in terms of voltage, or current, or power (both voltage and current). Conventional solid state amplifier circuits, including most audio power amplifier units (discrete or hybrid) and most integrated operational amplifiers, excluding some on-chip CMOS operational amplifiers which only have capacitive loading and don't need current drivability, despite of their power handling scales, all have emitter/source follower output stages and utilize the three-stage architecture as shown in FIG. 1. The first stage is a trans-conductance amplifier (voltage in—current out), followed by a trans-impedance amplifier (current in—voltage out) in the second stage (which provides the main voltage gain for the whole amplifier) and a voltage follower in the third stage which is a unity gain current amplifier and is usually configured by emitter/source follower pairs to drive a load. An ideal amplifier should linearly increase the strength of the input signals only in both voltage and current without adding any extra components, by using 100% power taken from a power supply. However, a real-world amplifier can not ideally achieve the performance of distortion-free and 100% power efficiency. Since IC operational amplifiers only differ from audio power amplifiers in the output power scale while their circuit topology and functionality are almost identical, only audio power amplifiers are presented hereafter for simplicity. However, all the facts and conclusions for audio power amplifiers mostly remain true for IC operational amplifiers unless specified.
The classification of a power amplifier usually depends on the biasing of the amplifier's output stage. Although Class D power amplifier reveals a new design technology and is getting attention for its high power efficiency in audio products, it still suffers from various problems like possible EMC (Electromagnetic Compatibility) emission issues and design difficulties due to its strong performance dependence on device characteristics. Class D power amplifier is also not suitable for high quality home audio products since it has to drive separate speaker systems which hardly have the required impedance characteristics. Therefore, most home audio amplifiers still use traditional design and the output stage of an audio power amplifier is usually biased at Class A, Class B, or Class AB. When the output stage is biased at Class B or Class AB for a compromise between performance and power/cost efficiency, total distortion of the three stages is usually dominated by the distortion generated by the last stage if the first and second stages are carefully designed. The last stage distortion results from two reasons: the power transistors alternative switching-on and switching-off functions during the full input signal cycle and the heavily curved V-I transfer function of the transistors in the small signal vicinity. The distortion is called crossover distortion and strongly quiescent bias current dependent. The output stage's crossover distortion is one of the most daunting distortions in audio power amplifiers, since it generates high order harmonics which significantly degrade sound/music reproduction quality even if it is not the dominant part among the whole distortion. Badly, the crossover distortion tends to increase when the output level drops, further worsens amplifiers' real performance since the volume most people listen is modest. Solid state power amplifiers have long been blamed for not sounding as smoothly and warmly as vacuum tube amplifiers and generating so called “transistor sound” by some audiophiles, which is believed highly due to the crossover distortion which intrinsically associates with Class B and Class AB amplifiers.
A common and effective way for reducing the crossover distortion and also improving other performance is to employ overall negative feedback (NFB). Almost all audio power amplifiers use overall NFB to achieve targeted performance, as illustrated in FIG. 1. As we all know, for a practical amplifier, we also need to implement compensation to retain stability when overall NFB is implemented. The most commonly used compensation technique is to add a Miller compensation capacitor to the second stage, as the Cf shown in FIG. 1 and the C1 shown in FIG. 2. Miller compensation capacitor is a small capacitor (10s of PF to 100 pF) that is connected between the input node and the output node of the second stage (which can be either a single common-emitter BJT amplifier or a cascoded common-emitter common-base BJT pair, or their MOSFET counterparts). The Miller compensation dramatically pulls the dominant pole towards the low frequency region (and also moves the second pole to higher frequency region as so called “pole-splitting”), so that the open loop gain at high frequency gets greatly reduced and hence the overall NFB won't cause stability problem. The Miller compensation actually provides a local feedback loop around the second stage so that its distortion and drivability is improved. The enhanced drivability of the second stage can also help reduce the third stage's crossover distortion somewhat, though this is only a subsidiary effect and far less adequate. Since the major distortion contributor, the output stage, stays outside of the local Miller NFB loop, further crossover distortion reduction only relies on the overall NFB. On the other hand, since the Miller compensation forms the dominant pole at a frequency point ranging from 10s of Hz to several kHz, the open loop gain begins to roll off from this low frequency and leaves much less NFB available to linearise the output stage (and the other stages as well) at higher frequencies, as illustrated in FIG. 3. This is the reason why conventional power amplifiers have bigger distortion at high frequency band. Almost all existing power amplifiers suffer from higher distortion from a frequency point through 100s of Hz to several kHz. Although it appears that lower distortion could be achieved with larger overall NFB by increasing open loop gain, however, larger overall NFB can reduce distortion at low frequency only, not efficiently at high frequency. This is because with a higher open loop gain, though more NFB can be implemented, more compensation is also needed accordingly. As a result, the dominant pole becomes lower and the open loop gain begins rolling off earlier, leaving the available NFB barely changed above the rolling-off frequency. Therefore, it is impossible to rely on overall NFB to further reduce distortion at high frequencies. On the contrary, modern audio power amplifiers tend to use overall NFB as little as possible to avoid or minimize TIM (Transient Intermodulation) distortion. TIM happens when the amplifier's slew rate is too low to follow fast pulse input. The slew rate is normally limited by the possible maximum slew current and the capacitance of the Miller capacitor. With the same slew current (which is usually close to the total bias current of the input stage), the bigger the Miller compensation capacitor, the lower the slew rate. With heavy overall NFB, bigger Miller compensation capacitor is inevitable and therefore the slew rate is further limited. So, increasing overall NFB is also not preferable when TIM distortion is concerned.
In order to achieve low distortion over the whole audio frequency range, the simplest solution to further reduce the output stage associated cross-over distortion is to increase its quiescent biasing current. The ultimate low distortion design is to bias the output stage at Class A, i.e., the upper/lower transistor pair constantly has current flow through in the full input signal cycle hence there is no switching operation and the upper/lower transistor pair works at a much more linear region. Combined with well designed input and driver stages, Class A audio power amplifier can achieve as low as 0.002% distortion from low frequency through 20 KHz. However, Class A power amplifiers have obvious drawbacks: it is very power inefficient, and since the output stage consumes the biggest power at idling, it generates large amount of heat. Therefore, a big heatsink is usually inevitable for Class A power amplifiers, which makes Class A power amplifiers bulky, heavy, and highly expensive. In addition to these disadvantages, Class A amplifiers, including highly biased Class AB amplifiers, need careful thermal design to prevent the hot output transistors from thermal runaway, which is especially true for BJT output devices since these devices normally have positive temperature coefficient. In order to reduce the power consumption while achieving performance equal to or close to what Class A amplifiers provide, intensive researches have been done in the past. One of the proposed solutions is to actively bias the final stage so that the power transistors never turn off, i.e., the upper/lower emitter/source follower transistors always have current flow through in either positive or negative input signal cycle so that it avoids the switching operation. Although this method can eliminate the switching associated distortion, it still suffers from the distortion associating with the curved V-I transfer function at small signal level. In addition, the active bias causes the concern of extra distortion due to the bias circuit since it is input related. Another idea is to adjust the power supply voltage level for the output stage in accordance with the input signal level and that the output transistors are biased at Class A with an input dependent varying power supply voltage, so that power consumption can be kept relatively low. This methods could improve the power efficiency while achieving Class A (or near Class A) performance. However, additional power circuit is needed to promptly adjust the power supply voltage for the output stage, which not only increases circuit complexity and cost, but also leads to the concern of the extra power consumption. There are also other approaches proposed to achieve low distortion for power amplifiers other than setting the final stage working at Class A biasing. One example is to use two amplifiers: one is high power/poor performance and the other is low power/high performance. The two outputs of the two amplifiers are summed through a well calculated network bridge, so that the distortion can be cancelled and hence only the pure amplified signal goes to the load. In order to achieve zero or very low distortion, a set of conditions need to hold true over all the frequency range and all the operational environments. However, in real-world practical applications, the variation of devices' aging parameters over life time and environments would make it difficult for the conditions to be maintained for all life time of the product. Also, this approach uses two amplifiers and the circuit is much more complicated, as compared to conventional designs.
Therefore, it is still a challenging task to design a crossover distortion free, low cost, light weight and power efficient power amplifier with existing technologies for commercial applications.